In offshore oil production, one utilizes a set of pipes that flows out the production of a productive well on the bed of the sea towards a platform that is located on the surface of the sea and the aforementioned set of pipes control valves and pumps located on the well-heads. This set of pipes (electrohydraulic, umbilical lines, injection of water and pumping of oil and gas properly spoken) is on a conventional basis, denominated production line.
This set of pipes that make up the production lines are basically subdivided into three different portions:
The first portion, the major part of which is horizontal and is laid on the seabed, is made up by a piping that may be either stiff or flexible, that connects the oil well to a point that stands under the location of the platform, and which is called horizontal section-of-pipe for collecting purposes. This section-of-pipe is known in the technical jargon as flow.
The second portion is a bended-shaped and suspended section-of-pipe that connects the piping that is essentially horizontal and stands on the seabed, with a third section-of-pipe that is mostly vertical. This section-of-pipe, which is typically curve-shaped and suspended, may have the configuration of a free catenary, comes to show configurations known as lazy-wave or steep-wave.
And the third portion is made up by a section-of-pipe that is mostly vertical and contiguous to the formerly mentioned piping, goes upwards until the surface, being denominated vertical section-of-pipe for collection purposes.
The set-of-pipes composed of the bended-shaped section followed by the mostly vertical portion is called riser in the technical jargon, and shall be treated conventionally as from this text as production collection line.
The production collection lines may either be stiff or flexible and can be deemed to be the most critical structures of a system of production in the sea (offshore).
These production lines usually undergo operational wear, such as the action of the outer and inner pressure, the innerside friction between the several layers that constitute it, the presence of corrosion, fatigue, in addition to undergoing incessant action derived from the dynamic and variable conditions related to the environment. The production collection lines are also submitted to the influence of the large-sized movements resulting from the floatability of the platform, the aforementioned movements taking place both in the horizontal and the vertical senses, on account of the tides, the sea currents, and the waves.
In summary, a production collection line may be undergoing several mechanical loads, such as axial traction applied on its higher end that is located on the platform; the very weight of the structure; in some cases, the thrust load on the liner; first- and second-order movements caused by the movements of the platform; the load that is derived from the waves; dragging force stemming from the sea currents and other hydrodynamic forces, like the one that takes place on account of vorticity; cyclical loadings that induce fatigue into the structure; and corrosion-connected effects caused both by the environment and the innerside fluid.
On the other side, when the production collection line is submitted to large-sized deflections, such as buckling, that takes place in the bended-shaped section-of-pipe of the catenary located next to the sea ground, its outer wall may show wrinkles (in the case of the fine wall thicknesses) or indentations (in the case of average to thick wall thicknesses). In the flexible-type production collection lines, the phenomenon of buckling may cause a defect known as “bird cage” in the steel wires that make them up.
As a highlight, one shall give attention to the hydrodynamic force that is derived from the known effect of loosening of vortices originating from the action of sea currents around the outer surface of the production collection line. The vibrations that are induced by vortices give origin to low-amplitude and high-frequency, which are transversal to the sense of the sea currents called “VIV” (Vortex Induced Vibrations). These loads may lead to a precocious fatigue process that, in its turn, may bring about the collapse of the pipe, causing a disaster of environmental nature and a high operational loss.
For these reasons, the production collection line is a critical element to the continuity of production and also for the safety of the environment. Because this one element is subject to the most diverse efforts that may simultaneously affect its structure, it is necessary that the whole production collection line be submitted to a stringent regular inspection.
There are currently some means with which it is possible to carry out these inspections. The main items verified in relation to the outer condition are the level of wearing of the cathodic production system, the existence of crackles or kneading and incrustrations, among others.
The exam of the structure is carried out through ultrasound, techniques based on induced magnetic fields, and visual exams. One also utilizes other kinds of Non-Destructive Testing (NDT), but each exam is performed by means of a specific procedure and specific pieces of equipment.
Most of these analyses are made at present by utilizing the PIGs (Pipe Inspection Gauge), which are introduced into the innerside of the production collection line and move along the section-of-pipe where one wants to carry out the inspection. Such gauges could be inserted to move forward by the production fluid itself. Each PIG is fitted with a specific inspection piece of equipment for the reading of ultrasound or any kind of other equipment that enables one to make analyses from the innerside of the production collection line.
In order to make inspections by means of this tool, it is necessary to order the stoppage of the production of the branch for the introduction of the PIG. The production of this branch is reduced to the minimum, or is even paralyzed, until the end of the inspection. As the time of stoppage is directly related to the speed of dislocation of the tool into the innerside of the pipe, which is around 1 km/h, one would suppose that a pipe measuring 20 km would require a stoppage of at least 20 hours, only in one sense of the dislocation of the tool.
In addition to limiting the inspection to the type of equipment that one supplies to it in order to carry out the analyses, the PIG does not perform an outer visual inspection of the structure of the production collection line.
The outer visual inspection is especially relevant when the structure of the production collection line is of the flexible type, because damage to the outer cover exposes to the severe environmental conditions the metallic wires that vest upon it a structural resistance.
Damages to the outer cover may be caused from abrasion, fall of platform-originated material onto the production collection line, and from growth of sea life on the outer cover.
At present, the outer visual inspection of the production collection lines is made by Remotely Operated Vehicle (ROVs). In addition to being a costly operation, the technician that operates the equipment must take extreme care in maneuvering the vehicle so as to circulate the perimeter of the production collection line along its whole length in order to ensure the analysis of the entire external surface of the structure, mainly the portion of the catenary that is subject to buckling and, therefore, is critical in relation to fatigue and cracklings.
Another parameter that needs to be monitored is the influence of the hydrodynamic force derived from the loosening of vortices, stemming from the action of the sea currents around the outer surface of the production collection line. Because it is a critical parameter that may accelerate fatigue, there must be a collection of data from the greatest number of points along the production collection line.
These data are currently collected by fitting the production collection line with bottles of Vortex Induced Vibration (VIV), that are accelerometers assembled in the innerside of metallic cylinders. These VIV-bottles are fixed on the production collection lines during their installation, or at any time by means of a ROV.
Because of the unitary cost of each bottle of VIV and because of the installation cost, only a certain number of these bottles is set up on the production collection line so that they may supply data from some representative points of the umbilical structure. The sets of information are stored into the internal memory of the bottles for a certain period of time. Later on, the VIV bottles are collected by ROV for the analysis of the data recorded therein.
When one wishes to have a greater number of data on the VIV, one also makes use of simulations performed in large-sized closed laboratories, where segments of production collection lines are exposed to currents of fluids that can be induced under control. This method, despite of furnishing the software of analyses with valuable data, does not supply real data in real time.
Taking into consideration what was presented hereinbefore, in terms of the techniques currently known for the inspection of production collection lines, there is no piece of equipment that allows for the making of ultrasound inspections or other non-destructive testing concurrently with the making of external visual inspection and that, in addition thereto, make it possible for one to collect data from VIV.
The inspections made by PIG do not allow for an outer visual inspection, and the collection of data originated from VIV is made in a certain number of fixed points. There is no piece of equipment that permits in loco to choose and to vary the points wherein one wishes to make the collection of VIV data on the production collection line and that supplies the data in real time for the purposes of analysis.
In order to overcome these problems of regular verification of the state of the production collection line, avoiding stoppages in the production and reducing the number of procedures and pieces of equipment involved, one has conceived an outer set of devices, the purpose of which is the universal inspection of risers.
The invention described hereunder is derived from the continuous research in this segment, the focus of which aims at eliminating the necessity of using PIGs, and also ROVs, so as to unify the various procedures into one single operation.
This invention is aimed at furnishing a device that may be utilized in any production collection line, reaching any depth at which one wishes to make an inspection, such a device being fitted with means of carrying out visual inspections that are concurrent with Non-Destructive Testing (NDT), and moreover to associate means of collecting data from the VIV at any point of the structure.
Other aims that the outer device for universal inspection of risers proposes to reach, which is the purpose of this invention, are listed hereunder:                (a) To eliminate the necessity of utilizing ROVs for the visual inspection, reducing both the cost and the time factors;        (b) To enable a visual inspection with a three-dimensional image;        (c) To eliminate the need of utilizing bottles of VIV in order to collect data from the operation of accelerometry, and as a consequence, the utilization of ROVs in the fixation of the bottles;        (d) To eliminate the necessity for PIGs in order to make NDTs;        (e) To ensure, in real time or offline, the reading of the data and of the images captured;        (f) Allow for the choice of the points from where one wants to record data for VIV;        (g) To allow for the making of NDTs without the need to interrupt production;        (h) To increase the efficiency of the inspection in the section-of-pipe of the catenary;        (i) To allow for the operations of cleaning of the outer wall of the production collection line; and        (j) To allow for X-ray analyses.        